Phase REFACTOR-3: Inline Pointer Macro Centralization (tiny_base_to_user_inline)

Centralize BASE ↔ USER pointer conversions into reusable, zero-cost macros.
Previously, pointer arithmetic (base + 1, ptr - 1) was scattered across
allocation/deallocation code with hardcoded offsets.

Changes:
- NEW: core/box/tiny_ptr_convert_box.h
  - tiny_base_to_user_inline(): BASE → USER (base + TINY_HEADER_OFFSET)
  - tiny_user_to_base_inline(): USER → BASE (user - TINY_HEADER_OFFSET)
  - TINY_HEADER_OFFSET: Centralized constant (currently 1)
  - Function variants: tiny_base_to_user(), tiny_user_to_base()

- Modified: core/front/malloc_tiny_fast.h
  - L181: return (uint8_t*)base + 1 → tiny_base_to_user_inline(base)
  - L299: void* base = (void*)((char*)ptr - 1) → tiny_user_to_base_inline(ptr)

Benefits:
- Self-documenting code (semantic intent is clear)
- Single source of truth for header offset
- Easier to extend (e.g., variable-length headers, alignment changes)
- Type-safe conversions (macro validates pointer types)
- Zero performance cost (inline macro, same compiled code)

Contract:
- Header stored at offset -1 from USER pointer
- Allocation: base → user (user = base + 1)
- Deallocation: user → base (base = user - 1)

No semantic changes - identical logic, just centralized.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

core/front/malloc_tiny_fast.h

@@ -49,6 +49,7 @@
 #include "../box/tiny_c6_ultra_free_box.h" // Phase 4-2: C6 ULTRA-free (free-only, C6-only)
 #include "../box/tiny_ultra_classes_box.h" // Phase REFACTOR-1: Named constants for C6/C7
 #include "../box/tiny_legacy_fallback_box.h" // Phase REFACTOR-2: Legacy fallback logic unification
+#include "../box/tiny_ptr_convert_box.h" // Phase REFACTOR-3: Inline pointer macro centralization
 #include "../box/tiny_front_v3_env_box.h" // Tiny front v3 snapshot gate
 #include "../box/tiny_heap_env_box.h"   // ENV gate for TinyHeap front (A/B)
 #include "../box/tiny_route_env_box.h"  // Route snapshot (Heap vs Legacy)
@@ -177,7 +178,7 @@ static inline void* malloc_tiny_fast(size_t size) {
             FREE_PATH_STAT_INC(c6_ultra_alloc_hit);
             // BASE pointer のまま、USER pointer に変換して返す
             // (header は既に base[0] にある前提)
-            return (uint8_t*)base + 1;
+            return tiny_base_to_user_inline(base);
         }
     }
 
@@ -295,7 +296,7 @@ static inline int free_tiny_fast(void* ptr) {
     }
 
     // 4. BASE を計算して Unified Cache に push
-    void* base = (void*)((char*)ptr - 1);
+    void* base = tiny_user_to_base_inline(ptr);
     tiny_front_free_stat_inc(class_idx);
 
     // Phase FREE-LEGACY-BREAKDOWN-1: カウンタ散布 (1. 関数入口)